Field of Invention
The present invention relates to a light emitting device current regulator circuit and a control method thereof; particularly, it relates to such light emitting device current regulator circuit and control method thereof with simplified wiring and low power loss.
Description of Related Art
FIG. 1 shows a conventional flat panel display (FPD) 100, which includes a display module 150 for displaying an image; a power management circuit 130, which converts an input voltage Vin to an output voltage Vout according to a feedback signal; and multiple light emitting device strings 110 for illuminating the display module 150. Each light emitting device string 110 includes multiple light emitting devices connected in series. One end of each light emitting device string 110 is coupled to the output voltage Vout for receiving operation power; the other end thereof is coupled to the power management circuit 130 for adjusting current flowing through the light emitting device string 110, and generating the feedback signal accordingly. In some applications, the brightness of the light emitting device string 110 is adjustable; in such case, the power management circuit 130 receives a dimming signal Dim, and adjusts the brightness of the light emitting device strings 110 according to the dimming signal Dim.
The power management circuit 130 receives multiple current sense signals, such as twelve current sense signals CS1, CS2, CS3, . . . , and CS12 shown in FIG. 1, and controls current flowing through each light emitting device string 110 according to the twelve current sense signals individually.
In the aforementioned conventional FPD 100, each light emitting device string 110 needs to be coupled to the power management circuit 130 individually. The larger the size of the FPD 100 is, the more the light emitting device strings 110 are needed in number, and so are the number and length of wires required for connection. This means more complicate wiring and more space in need. For example, as shown in FIG. 1, 12 light emitting device strings 110 require 12+1 wires. Besides, if the light emitting devices are connected in series in one light emitting device string 110 by a larger number, a higher operation voltage is required, which leads to higher manufacturing cost and safety concern. Furthermore, when the number of the light emitting device string 110 or the number of the light emitting devices in one light emitting device string 110 changes, the power management circuit 130 and the wiring need to be modified correspondingly. These changes and modifications lead to a higher manufacturing cost.
FIG. 2 shows a schematic circuit of a light emitting device control circuit 200, which is filed by the same applicant as U.S. Ser. No. 13/096,421 on Apr. 28, 2011. As shown in the figure, the light emitting device control circuit 200 includes a power supply circuit 270, multiple light emitting device strings 210, and multiple light emitting device current regulator circuits 230. The power supply circuit 270 converts an input voltage Vin to an output voltage Vout according to a feedback signal FB. Each light emitting device strings 210 includes at least one and preferably multiple light emitting devices connected in series. The light emitting device string 210 has a first end E1 and a second end E2, wherein the first end E1 is coupled to the output voltage Vout to supply operation power to the multiple light emitting devices. The light emitting device current regulator circuit 230 has pins Vcc, CS, LFB, and GND. Pin Vcc receives an operation voltage which is supplied to the internal circuitry of the light emitting device current regulator circuit 230 (hereinafter referred to as internal voltage Vcc). The internal voltage Vcc is converted from the output voltage Vout or other proper power sources, such as the input voltage Vin or other DC voltages. Pin CS of the light emitting device current regulator circuit 230 is coupled to the second end E2 of the light emitting device string 210 to regulate current flowing through the light emitting device string 210. The light emitting device current regulator 230 generates a local feedback signal LFB at pin LFB. All local feedback signals LFB generated by the multiple light emitting device current regulator circuits 230 are coupled to a feedback signal pin FB of the power supply circuit 270 to provide a feedback signal FB. The feedback signal FB is determined by the lowest level among the multiple local feedback signals LFB. In some applications of the light emitting device control circuit 200, the brightness of the light emitting device strings 210 is adjustable; in such case, the light emitting device current regulator circuit 230 may include a pin Dim to receive a dimming signal Dim. The light emitting device current regulator circuits 230 receive the same dimming signal Dim, and regulate current flowing through corresponding light emitting device strings 210 according to the dimming signal Dim.
In FIG. 2, because the light emitting device current regulators 230 are provided locally and connected with the corresponding light emitting device strings 210 to become one local module, the wiring is simplified as compared with the prior art of FIG. 1. The number of the wires is greatly reduced to four, including: an output voltage common wire for delivering the output voltage Vout; a feedback signal common wire for delivering the feedback signal FB (LFB); a ground common wire for connection to ground level GND; and a dimming signal common wire for delivering the dimming signal Dim. In the prior art shown in FIG. 1, if there are N light emitting device strings 110, N+1 wires are needed. In contrast, the circuit shown in FIG. 2 obviously saves space effectively. Besides, in the prior art shown in FIG. 1, for different number of light emitting device strings 110, the internal circuitry and the number of pins of the power management circuit 130 need to be modified or re-designed. In the circuit shown in FIG. 2, the power supply circuit 270 can be used to cooperate with any number of light emitting device strings 210 without changing its internal circuitry or the number of pins, as long as the total power required does not exceed the limit. Therefore obviously, the circuit shown in FIG. 2 is more advantageous than the prior art.
However, even though the circuit shown in FIG. 2 simplifies the wiring as compared to the prior art shown in FIG. 1, it is still required to provide the internal voltage Vcc to each light emitting device current regulator circuit 230 by an additional common wire. Hence, the wiring and the power loss problems can be further improved.
In view of the foregoing, the present invention provides a light emitting device current regulator circuit and a control method thereof, which can further simplify the wiring and mitigate the power loss.